This disclosure is related to the field of milling tools used to remove objects from a wellbore other than formations to be drilled. More particularly, the disclosure relates to mills that may be used in wellbores completed using multiple stage fracture treatments prior to configuring the wellbore for production as well as mills for scale removal/cleanouts, casing exits, etc.
In the process of fracture treatment of a well that penetrates a formation over an extended axial distance, the fracture treatment may be performed along separate axial intervals in successive stages. Various equipment is used to isolate each fracture treatment stage and that equipment creates restrictions inside the completed casing. In many cases, it is desirable to mill away the stage isolation equipment to allow as large a bore as possible (full casing/liner ID) to enhance hydrocarbon recovery. Up until very recently, most multiple stage fracture treatments included up to around 25 stages. The stage isolation equipment in such wellbores is typically milled out with either conventional junk mills (simple products with crushed carbide or sharp carbide inserts held in by weld), roller cone drill bits, and much less frequently with polycrystalline diamond compact (PDC) mills.
The foregoing milling operations are typically performed using an hydraulic motor deployed in the wellbore at the end of a coiled tubing. Such operations are also conducted with small, sometimes truck mounted rigs with conventional drill pipe as well
Such milling operations are generally performed in small internal diameter well casings (e.g., 3.5 inches to-4.75 inches), and, therefore they use small diameters hydraulic motors that do not generate high torque. Stalling of such motors is very common and is a major concern when milling fracture stage isolation equipment as well as during other milling operations such as mills for scale removal/cleanouts, casing exits, etc. The inherent risk of motor stalling is one reason why junk mills and roller cone bits are preferred over PDC mills. Another reason the PDC cutter mills are not frequently used is that PDC cutters are often damaged because the materials used in the various parts of the fracture stage isolation equipment are of varying strength and are not consistently spaced within the cross-section of the interior of the wellbore casing. This results in high instantaneous loads on the mill, which may easily break PDC cutters.
In the process of milling out fracture stage isolation equipment it is important to the economics of the well that all the stage isolation equipment is milled in one milling operation, and such operation should be completed as quickly as possible. Therefore, using PDC mills known in the art, which are subject to damage as explained above, is not considered reliable. However, it has been observed that PDC mills can mill through the zone isolation equipment much faster than junk mills or roller cone bits. Roller cone bits generally work well, but due to the fact that most small diameter motors operate at a high RPM, and to the fact that small diameter roller cone bits have very small bearings, the rate of failure of seals & bearings on roller cones is relatively high. Roller cones are also a fairly expensive option for this type of work as they are a consumable (i.e., discarded after the run). Crushed carbide mills and PDC mills can be repaired, so the cost can be spread out over multiple operations to make the use of such mills more economical. Therefore, as with PDC mills, roller cone bits may not always be desirable due to the risk of premature failure.
More recently, multiple stage fracture treatment systems are being developed that include many more stages than using multiple stage systems known in the art. Such newer systems may include up to 100 stages in one lateral interval, and, in many cases, such systems include stage isolation devices that may use “drop balls” that are metallic, as contrasted with drop balls that are used in earlier multiple state fracturing systems made from composite materials. As a result, there is a need for a wellbore mill that can reliably and consistently mill wellbore devices and/or scale accumulation in one run, and complete such milling operation in an economically beneficial amount of time.